linux-kernel - Re: Possible issue with dangling PCI BARs

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Date:	Fri, 14 Dec 2007 07:51:06 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Jesse Barnes <jesse.barnes@...el.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Robert Hancock <hancockr@...w.ca>,
	linux-pci@...ey.karlin.mff.cuni.cz,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Possible issue with dangling PCI BARs

On Thu, 2007-12-13 at 12:04 -0800, Jesse Barnes wrote:
> 
> Yeah, that seems like a reasonable compromise.  Though in practice
> I'd 
> expect the full disable decode approach to work fairly well too.  I 
> mean, if we really end up failing to allocate space for the device
> with 
> the root drive on it, there are probably bigger issues than just 
> failing to get a few bytes of I/O space for it...
> 

The really bad scenario would be something like the Sil680 that Alan
talked about setup by a BIOS that "knows" about the unused BAR when
MMIO_EN is not set.

If the device is behind a P2P bridge and the BIOS has set the windows of
that bridge so tightly that there is no room to allocate the MMIO BAR,
then a full disable/full enable would fail on a device that would
otherwise work using only PIO.

However, I'd be curious to see that happening in practice :-)

But I think it's fair enough to do an IO only / MEM only approach. I've
seen cases where IO is just not useable because of other constraints and
so I expect the MEM-only case to be more common, especially on non-x86.

Ben.

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